Acoustic transducers generally convert electrical signals to acoustic signals (sound waves) and convert received acoustic waves to electrical signals via inverse and direct piezoelectric effect. There are a number of types of acoustic transducers including acoustic resonators, such as bulk acoustic wave (BAW) resonators and surface acoustic wave (SAW) resonators. BAW resonators, in particular, include thin film bulk acoustic resonators (FBARs), which generally have acoustic stacks formed over a substrate cavity, and solidly mounted resonators (SMRs), which generally have acoustic stacks formed over an acoustic mirror (e.g., a distributed Bragg reflector (DBR)). BAW resonators may be used for electrical filters and voltage transformers, for example, in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices.
Generally, a BAW resonator has an acoustic stack comprising a layer of piezoelectric layer between two conductive plates (e.g., top and bottom electrodes). The piezoelectric layer may be a thin film of various materials, such as aluminum nitride (AlN), zinc oxide (ZnO), or lead zirconate titanate (PZT), for example. Piezoelectric thin films made of AlN are advantageous since they generally maintain piezoelectric properties at high temperatures (e.g., above 400° C.). Indeed, BAW resonators have experienced mainstream adoption and success in wireless communications due in large part to the characteristics of thin film ALN piezoelectric layers. However, a BAW resonator including a piezoelectric layer formed of AlN has a resonance frequency limited to less than about 3 GHz, as a practical matter, in order to maintain acceptable device performance and reliability.
Thin film AlN is typically grown in a c-axis orientation perpendicular to a substrate surface using reactive magnetron sputtering. In this process, an aluminum (Al) target is sputtered by inert gas atoms, such as argon (Ar) atoms, in the presence of nitrogen (N) gas. Oriented, stoichiometric and piezoelectric AlN grows on any number of substrates ranging from silicon (Si) and silicon carbide (SiC) to metals, such as molybdenum (Mo) and tungsten (W), for example. An AlN thin film may be deposited with various specific crystal orientations, including a wurtzite (0001) B4 structure, for example, which consists of a hexagonal crystal structure with alternating layers of aluminum (Al) and nitrogen (N). The piezoelectric nature of AlN stems from the c-axis orientation and the nature of the Al—N bonds of the AlN crystal lattice. That is, due to the nature of the Al—N bonding in the wurtzite structure, electric field polarization is present in the AlN crystal, resulting in the piezoelectric properties of the AlN thin film To exploit this polarization and the corresponding piezoelectric effect, one must synthesize the AlN with a specific crystal orientation.
FIGS. 1A and 1B are perspective views of illustrative models of common wurtzite structures of piezoelectric materials. Generally, for purpose of discussion, polarization of a piezoelectric material is defined as being in the “positive direction” from cation (e.g., Al atoms) to anion (e.g., N atoms) along the crystallographic axis points. Accordingly, as shown in FIG. 1A, when the first layer of the crystal lattice 100A is an Al layer and second layer in an upward direction (in the depicted orientation) is an N layer, the piezoelectric material including the crystal lattice 100A is said to have “positive polarity,” as indicated by the upward pointing arrow 150A. Conversely, as shown in FIG. 1B, when the first layer of the crystal lattice 100B is an N layer and second layer in an upward direction is an Al layer, the piezoelectric material including the crystal lattice 100B is said to have “negative polarity,” as indicated by the downward pointing arrow 150B. Notably, the orientation shown in FIG. 1B is the more standard convention in the field of polar nitride materials. A piezoelectric material having a single polarity (positive or negative) is limited in the resonance frequency at which it is able to perform without compromising various characteristics, such as coupling coefficient kt2, for example.